Traditional spectroscopic methods are limited in sensitivity to approximately one part per ten thousand (1:10.sup.4) to one part per hundred thousand (1:10.sup.5). The sensitivity limitation arises from instabilities in light source intensity that are translated into noise in the absorption signal. For general information on traditional spectroscopy methods see for example Dereniak and Crowe, Optical Radiation Detectors, John Wiley & Sons, New York, 1984, and Demtroder, Laser Spectroscopy, Springer, Berlin, 1996.
Cavity Ring-Down Spectroscopy (CRDS), a technique first described by O'Keefe and Deacon in an article in Rev. Sci. Instrum. 59(12):2544-2551 (1988), allows making absorption measurements with sensitivities on the order of one part per ten million (1:10.sup.7) to one part per billion (1:10.sup.9) or higher. For general information on CRDS see U.S. Pat. No. 5,528,040 by Lehmann, herein incorporated by reference, as well as the articles by Romanini and Lehmann in J. Chem. Phys. 102(2):633-642 (1995), Meijer et al. in Chem. Phys. Lett. 217(1-2):112-116 (1994), Zalicki et al. in App. Phys. Lett. 67(1):144-146 (1995), Jongma et al. in Rev. Sci. Instrum. 66(4):2821-2828 (1995), and Zalicki and Zare in J. Chem. Phys. 102(7):2708-2717 (1995).
In a conventional CRDS system, the sample (absorbing material) is placed in a high-finesse stable optical resonator consisting of two spherical mirrors facing each other along a common optical axis. Light incident on one mirror circulates back and forth multiple times in the resonator, setting up standing waves having periodic spatial variations. Light exiting through the other mirror measures the intracavity light intensity.
The radiant energy stored in the resonator decreases in time (rings-down). For an empty cavity, the stored energy follows an exponential decay characterized by a ring-down rate that depends only on the reflectivity of the mirrors, the separation between the mirrors, and the speed light in the cavity. If a sample is placed in the resonator, the ring-down is accelerated; under suitable conditions, the intracavity energy decays almost perfectly exponentially. An absorption spectrum for the sample is obtained by plotting the reciprocal of the ring-down rate versus the wavelength of the incident light.
CRDS has been applied to numerous systems in the visible, ultraviolet, and infrared. For information on the use of CRDS for spectroscopy in the visible, see the articles by Engeln and Meijer in Rev. Sci. Instrum. 67(8): 2708-2713 (1996), Martin et al. in Chem. Phys. Lett. 258(1-2):63-70 (1996), Paul et al. in J. Chem. Phys. 104(8):2782-2788 (1996), Scherer et al. in J. Chem. Phys. 103(21):9187-9192 (1995), Scherer et al. in J. Chem. Phys. 102(13):5190-5199 (1995), Scherer et al. in Chem. Phys. Lett. 242(4-5):395-400 (1995). Heustis et al. in Canadian J. Phys. 72(11-12):1109-1121 (1994), and O'Keefe et al. in Chem. Phys. Lett. 172(3-4):214-218 (1990). Information on CRDS applications in the ultraviolet can be found in the above-referenced articles by Romanini and Lehmann, and Zalicki et al., as well as the articles by Zhu et al. in Chem. Phys. Lett. 257(5-6):487-491 (1996), Romanini and Lehmann in J, Chem. Phys. 105(1):81-88 (1996), Romanini and Lehmann in J. Chem. Phys. 105(1):68-80 (1996), Wahl et al. in Diamond and Related Materials 5(3-5):373-377 (1996), Boogaarts and Meijer in J. Chem. Phys. 103(13):5269-5274 (1995), Zalicki et al. in Chem. Phys. Lett. 234(4-6), 269-274 (1995), Jongma et al. in J. Molecular Spectroscopy 165(2):303-314 (1994), and Romanini and Lehmann in J. Chem. Phys. 99(9):6283-6301 (1993). For information on the use of CRDS for infrared spectroscopy see the above-referenced article by Martin et al., as well as the article by Scherer et al. in Chem. Phys. Lett. 245(2-3):273-280 (1995).
In comparison to conventional spectroscopy techniques, CRDS is advantageous because of the increased pathlength due to multiple reflections. CRDS is also advantageous because of its relative insensitivity to variations in the amplitude of light generated by the light source. In a CRDS system, fluctuations in the intensity of the light source do not typically limit sensitivity. Conventional CRDS systems are subject to optical feedback from the resonator to the light source, however. As long as the linewidth of the resonator is much narrower than the laser linewidth (as in most systems), only a small fraction of the light incident on the resonator enters the resonator; most of the light is reflected back toward the laser. Optical feedback to the laser leads to excess noise in its frequency tuning, mode oscillation stability, and power output. The excess laser noise leads in turn to unstable laser-resonator coupling, increased baseline noise, and reduced absolute sensitivity for absorption measurements.
Optical feedback from a resonant cavity to a laser is of concern for many application using linear or folded resonant cavities. For examples of non-CRDS spectroscopy systems see for example U.S. Pat. Nos. 5,173,749 and 5,432,610. A method of reducing optical feedback to the laser would find use in many non-CRDS spectroscopy applications.